Rapid expulsion is the major protective mechanism by which rats control challenge infections with T. spiralis. These studies will investigate the immunobiology of the reaction by defining: 1) The nature of the functional immune response to preadult T. spiralis. 2) The larval stichosome antigens against which immunity is directed and the time and site of initiation of rapid expulsion. 3) The changes that take place in the intestine during intestinal stimulation, during rapid expulsion and in the resulution phase after rapid expulsion has occurred. In addition comparative studies will be undertaken to determine the generality of the rat rapid expulsion to other host species (mouse) and other parasites (S. rattii).